(a) Field of the Invention
The present invention relates to a Gauss type lens system and, more particularly, to a low-price Gauss type lens system with the aperture ratio of 1.8 and field angle of 46.degree. or more.
(b) Description of the Prior Art
As a known Gauss type lens system with the aperture ratio of 1.8 and field angle of 46.degree. or more, there is a lens system disclosed, for example, in Japanese published examined patent application No. 41339/71. One of characteristics of this lens system is that its back focal length is long and the actual back focal length of this lens system is 0.72 f. In case of this lens system, however, the thickness of cemented doublet on the object side is large and, therefore, the back focal length is limited even when it is made long. Moreover, as the thickness of cemented doublet on the object side of this lens system is large, the intensity of light in the marginal portion becomes insufficient. Moreover, the material cost becomes high because glass materials with high refractive indices are used for the first lens component, lens element on the image side in the cemented doublet arranged on the image side, and lens component nearest the image side. To prevent the high material cost, it is all right when high refractive-index high-dispersion glass materials are used. In case of the known lens system disclosed in Japanese published unexamined patent application No. 140322/77, high refractive-index high-dispersion glass material is used for the first lens component, which is a positive lens. However, in case of this lens system, the aperture ratio is 1:2 and, therefore, it is comparatively easy to obtain a lens system with favourably corrected aberration. Moreover, there is high degree of freedom in the selection of glass materials for the lens element on the image side in the third lens component and for the fourth lens component.